1. Field of the Invention
This invention relates to an electronic private branch exchange (PBX) in which a calling rate can be economically selected at the time of placing a call over an office line, that is, a line connecting the PBX to an outside switching network.
In a prior art electronic private branch exchange connected to a public telephone line and an exclusive line, office line trunk circuits (hereinafter merely called trunk circuits) which connect the telephone exchange to the PBX are classified into a plurality of groups in accordance with the difference in the signal type of the circuits, rate system, and dial numbering system. The order of priority for connecting callers to the trunk circuits in the PBX network is set for the trunk circuits by respective groups, and a trunk circuit connectable to the office line is detected in accordance with the order of priority for connecting the lines. In such a PBX, a calling rate can be economically selected by setting the order of connection according to the amount of rate to be paid.
A Bell telephone exchange network developed in the United States of America has been well known as this type of PBX. According to this network, there are provided exchange line networks of different rate classes which are termed "trunk lines" to be charged at different rates, which includes full business day (FBD) and wide area telephone system (WATS) networks in which the telephone can be used in a wide area at a single rate system, a message toll (MT) WATS network utilizing different rate systems, and a direct line distance dial (DDD) network utilizing another rate system. Consequently, the overall calling charges reduced when the trunk circuit connected to the office line is selected from the lowest rates available.
In the Bell telephone network, when a call from an office line is attempted, an idle trunk circuit in a FBD network which is the lowest rate is detected by sequential scanning, whereas when there is no idle trunk circuit in the FBD network, the scanning is continued until an idle trunk circuit is detected in the MT network.
Other types of idle trunk circuit scanning systems which are known include a waitless trunk limitation system, a trunk limitation waiting system and a definite time waiting system. In all of these systems, an idle trunk circuit is sequentially detected according to the lowest available rate.
In the above described idle trunk scanning systems, however, all trunks circuits, in priority, are checked for availability. Therefore, where large numbers of calls are being placed, it takes a long time until an available idle trunk circuit is detected, whereby a calling party waits a long time. Otherwise, the calling party would be connected to a trunk circuit of a high rate against his will. Furthermore, a control device for detecting an idle trunk circuit must continue the sequential scanning of respective trunk circuits until an idle trunk circuit is detected, which increases the burden of the control device and further increases the waiting time of the calling party.